A correlation method for detection of partial discharges in transformers

Frequency domain correlation technique for PD location in transformer winding

Abstract: Partial discharge (PD) is one of the sources of insulation failures in Power transformers. Knowledge of the PD source locations are important for transformer maintenance and repair. Due to the complex structure of the transformer, it is very difficult to locate the PD accurately in transformer winding. Correlation methods are applied to locate the PD source across the winding. The PD across parts of the winding are simulated by applying voltage pulse across the sections. The winding response for a reference PD impulse across transformer sections are taken as base for correlation and correlated with winding response of PD impulse across any section with different PD pulse widths. In this paper the limitation of time domain correlation method in locating PD in transformer windings is found out. To overcome the limitation of time domain correlation method, frequency domain correlation with Gram Schmidt orthogonalization is proposed. Simulation and experiments are performed on layer, continuous disc and interleaved windings to prove the feasibility of the proposed method.

Virtual Instrument for Lightning Impulse Tests

Abstract: We propose an objective formulation of the impulse analysis problem from a signal analysis viewpoint. The winding response is quintessentially that of a deterministic network to a finite energy signal, with breakdown and partial discharge being inherently nonlinear events. A significant improvement to the acquisition of waveforms is demonstrated by a virtual instrument approach. It retains the advantages of the time- and frequency-domain methods. The drawbacks of the transfer function method are highlighted and a new piecewise linear approach is proposed for analysis. Experiments on a discrete lumped parameter model of the winding are used to validate the PXI based instrument.

Partial Discharge Localization in a Power Transformer: Methods, Trends, and Future Research

Abstract: This paper presents a comprehensive review of various techniques of partial discharge (PD) localization. The technologies reviewed are acoustic, ultra-high frequency (UHF), optical, and electrical according to their chronological order of development. The paper outlines the distinctive acoustic and UHF sensors, along with the algorithm for the localization of PD source. Various digital signal processing and statistical techniques applied are likewise discussed. A substantial effort has been given to electrical methods since they represent the most active and current field of PD research. In this paper, the techniques of PD localization used so far are compared, and their advantages and limitations are pointed out. The challenges and trends in future research in PD localizations are also discussed. The review given in this paper will be useful to develop the future technologies for PD localization thereby avoiding insulation damage. Thus, this paper is intended to serve as a guide for the research ...

Localization of Partial Discharges Inside a Transformer Winding Using a Ladder Network Constructed From Terminal Measurements

Abstract: Precise localization of partial discharge (PD) inside a power transformer winding is a challenging task. Previously, researchers have used internal calibration or reference signals to locate the PD source. The location of any arbitrary (test) PD source is ascertained from the maximum correlation between reference and test signals. However, in practical transformer windings, internal tappings or design details are usually unavailable to generate reference signals. The proposed work employs terminal measurements to construct a physically realizable ladder network. Simulated responses obtained from the ladder network for signals of known pulse-widths at all locations are used as reference data. The terminal responses of the test PD signals are obtained from a laboratory-scale winding by applying signals of arbitrary pulse-widths and shapes at various locations. The PD test signals are generated using a function generator, a PD calibrator, and real discharges. To predict the location of the PD source, the simulated reference data are then correlated with the test data. The position corresponding to the maximum correlation indicates the PD location. The proposed methodology is verified using experimental investigations carried out on two different laboratory-scale transformer windings.

Wavelet power ratio signature spectrum analysis for prediction of winding insulation defects in transformer and shunt reactor

Abstract: The reliable analysis for identification of winding insulation defects in transformer and shunt reactor are necessary for power industries during lightning impulse test. In this paper, wavelet power ratio signature spectrum analysis is proposed to identify the short duration of partial discharge (PD) pulses of 170 ns and 30 ns within windings due to impulse voltage excitation. It provides a solution to the problems associated with several random features in the measured winding responses at neutral terminal of the winding due to PD pulse through visual assessment of mutual strength between normal winding response and one with PD response using appropriate mathematical formulation in wavelet transform. In this context, the normalized response due to reduced impulse voltage excitation is formed to be a basis function to identify the PD pulse. If the surge impedance characteristic of the winding is changed due to PD pulse at full wave impulse voltage excitation, then formulated wavelet power ratio signature spectrum analysis will extract the evidence of potentially interesting features through assessment between basis function response and one with PD response. To prove feasibility of the proposed analysis, experimental analysis is performed on a layer winding.

Detection of signals in noise

Abstract: Probability. Random Processes. Narrowband Signals. Gaussian Derived Processes. Hypothesis Testing. Detection of Known Signals. Detection of Signals with Random Parameters. Multiple Pulse Detection of Signals. Detection of Signals in Colored Gaussian Noise. Estimation of Signal Parameters. Extensions. References. Bibliography. Index.

Detection and location of internal defects in the insulation of power transformers

Abstract: With the development of fast, and low-cost analog-digital converters (ADCs) and new mathematical procedures for waveform description, e.g., time encoded signal processing and recognition (TESPAR), novel approaches to detect and locate internal defects in the insulation of power transformers become feasible if the following requirements are fulfilled: the equivalent circuit of the insulating system, suitable for the fast partial discharge (PD) signals with risetimes of approximately 10 ns, is known; the response of the insulating system to PD signals injected at least at three different sites of each winding (top, center, bottom) is calculated, or measured at the signal tap-off points (i.e., bushings); and the pattern matrix of all calculated or measured signals exists: e.g., encoded and signal processes with TESPAR as a reference. The above mentioned requirements are examined for quasi-simultaneous detection and location of internal defects in power transformers. The method is applied to PD measurements on power transformers. >

Application of digital filtering techniques to the determination of partial discharge location in transformers

Abstract: The capacitative network representation of transformer windings has long been used for the study of pulse distribution along the windings. However, the author's test results and those published in the literature show that theoretical analysis based on this equivalent network may not be applicable. It was found that the capacitive network approximations were valid only within certain ranges of frequency. The frequency ranges were determined through the analysis of terminal measurements. The capacitatively transferred components of the input pulse along the windings in the appropriate frequency range were then extracted by the aid of digital filtering techniques, and good agreement between measured and theoretical distributions was obtained. These techniques, in conjunction with a new straight-line interpolation method, were employed to locate simulated partial discharges in a 66 and 132 kV transformer winding. Measurement results showed that an accuracy of better than 5% of winding length was obtained. >

Partial Discharge Location in Power Transformers

Abstract: In a power transformer, high levels of partial discharge activity may lead to an insulation failure. If a partial discharge (PD) could be detected and located before the insulation fails, the transformer could be repaired or replaced before any consumers connected to the transformer lose supply. A technique is described in the paper that can be used to locate a PD in a power transformer with a continuous disc type winding. The technique requires knowledge about the physical geometry and design parameters of the transformer and also information about the characteristics of how the pulse or pulses caused by the PD propagate through the winding. The latter is difficult to obtain and consequently numerous simulation studies with slightly different assumptions were necessary before the technique could be considered validated. This involved analysing the data available at the terminals when a PD occurred in the winding and trying to extract the features related to the PD location. An algorithm using feature template matching has been used to locate the PD source in a 110 kV continuous disc type winding. Experimental tests carried out on this type of winding were extremely important in verifying the validity of the PD location algorithm. Results indicate that a location accuracy of better than 5% of the winding length is normally achieved.

Location of partial discharges in transformers

Abstract: A matched filter approach is proposed for the location of a partial discharge within a winding. The approach is very general and does not need isolation of the capacitively transferred components. Simulations are performed on a lumped parameter model of a layer winding to prove the feasibility of the method. Experiments are also performed on a layer winding in order to validate the method. It is possible to extend the method to the determination of location of PD during impulse tests.